Test order of update application after Send

- updates from inside Send tasks are applied in the order the Sends were created, if when you fan out, and have each task write results to a list with reducer, the final list is in the order you used when triggering

libs/checkpoint-postgres/langgraph/checkpoint/postgres/base.py

@@ -84,7 +84,7 @@ select
             and cw.checkpoint_id = checkpoints.checkpoint_id
     ) as pending_writes,
     (
-        select array_agg(array[cw.type::bytea, cw.blob] order by cw.idx)
+        select array_agg(array[cw.type::bytea, cw.blob] order by cw.task_id, cw.idx)
         from checkpoint_writes cw
         where cw.thread_id = checkpoints.thread_id
             and cw.checkpoint_ns = checkpoints.checkpoint_ns